1. Field of Invention
The present invention relates to structures of electroluminescent devices used for terminal components of information apparatuses, such as displays.
2. Description of Related Art
Among recent aggressive activities for developing next generation luminous displays to replace cathode ray tubes (CRT) and liquid crystal displays, research and development of plasma display panels (PDP), field-emission displays (FED), organic electroluminescent displays have been actively performed. In organic electroluminescent materials, organic polymer materials emitting blue, green, and orange light, have been developed to such a level that the initial characteristics thereof can be utilized (The Society of Fiber Science and Technology, Japan, Symposium Abstracts 1998, 3A11, etc.). Polyfluorene derivatives have been well known as polymer materials for blue light emission, as described in Japanese Journal of Applied Physics, Vol. 30, No. 11B, November, 1991, pp. L1941–L1943. In addition, as light-emitting materials having wavelengths of green or longer, poly(p-phenylenevinylene) derivatives have been well known, as described in U.S. Pat. No. 5,247,190.
Meanwhile, in electroluminescent devices using light-emitting materials having low molecular weights, it has been reported in Appl. Phys. Lett., 70, 152 (1997) that electron injection efficiency was improved by providing a cathode interface layer.
However, concerning organic polymer materials for blue light emission, even though the initial characteristics thereof can be satisfactory, there is the problem in that the wavelength of the luminescent color shifts toward the longer wavelength side with current-application time.
In addition, in electroluminescent devices using organic polymer materials as light-emitting materials, impurities exist therein due to difficulties in purifying organic polymers, and current which does not contribute to light emission flows through these impurities, whereby there is a problem in that sufficient efficiency cannot be obtained.
Furthermore, in a process for manufacturing electroluminescent devices, printing defects may occur when a printing method, specifically, an ink-jet method, is used as a method for forming a light-emitting layer, and electrical short circuits caused by the printing defects may occur, whereby there is a problem in that display may be disabled.